Heavy load start and deceleration – High inertia

In applications with heavy loads and high inertia, such as long conveyors, crushers, or fully loaded extruders during startup, as well as centrifuges and compressors, a large starting torque is often required. This can easily cause the inverter to trigger overcurrent faults and usually necessitates extending the startup time or even selecting an inverter with 2 to 3 times higher power capacity, which increases the initial investment cost.

An alternative method to start heavy loads without increasing the inverter power rating too much is to divide the startup process into two stages. In stage 1, the inverter generates a high torque to pull the load up to a certain frequency and holds that frequency for a period of time. Then, in stage 2, the inverter accelerates the load to the reference frequency.

This startup method can be applied to other types of inverters either by using built-in functions or by programming additional logic with the PLC inside the inverter. If the inverter does not have this capability, an external PLC can also be used.

In this article, the example used is the DWell function of the Yaskawa GA700 inverter, which allows heavy load starting and deceleration for loads driven by asynchronous motors. When operating with a PM motor in V/f mode, the speed-holding process helps align the rotor of the PM motor with the stator’s magnetic field, thereby reducing the starting current.

• b6-01: the reference frequency at which speed holding begins
• b6-02: the duration of holding the reference speed
• Similarly for b6-03 and b6-04.

The issue of load balancing

We often encounter this issue when using two motors to drive a common load, such as overhead cranes, bridge cranes, cable winches, elevators, or tower crane turntables…

When the Droop Control function of the inverter is enabled, the inverter compares the actual torque with the reference torque. If the actual torque is greater than the reference, it means that this motor is under a heavier load than the other motor, so the inverter automatically reduces the speed to balance the load between the two motors. Conversely, if the actual torque is less than the reference, the inverter automatically increases the speed.

Note that we should only enable the Droop Control function on one of the two inverters, not both. Also, when the Droop Control function is enabled, the Feed Forward function must be turned off (n5-01 = 0).